1. Field of the Invention
The present invention relates to a polishing apparatus in which polishing of a workpiece is effected using dielectric abrasive particles located at a position at which processing pressure is applied by a Coulomb force produced when a voltage is applied across electrodes.
2. Description of the Prior Art
Polishing apparatuses having various configurations have been proposed. A polishing apparatus generally has a polishing pad holding abrasive particles that is affixed to a surface plate, and effects polishing by moving one of the plate and the workpiece with respect to the other. In recent years, progress has been made with research into functional fluids that respond to an electrical field or a magnetic field, and there are polishing apparatuses that utilize such functional fluids.
In processing such as polishing and surface finishing, for example, use has been made of magnetic fluids, which are fluids containing a dispersion of angstrom-order magnetic particles. When used on their own, such a fluid has almost no polishing effect, so abrasive particles are added to the fluid for polishing applications. The magnetism induced in the magnetic fluid by a magnetic field causes the abrasive particles to be pressed against the workpiece surface by the fluid. Although polishing using a magnetic fluid is suitable when the workpiece to be polished has a spherical or other such special shape, magnetic fluid polishing has a number of problems. For example, magnetic fluid induction produces a small processing pressure that results in a low polishing efficiency. In addition, a magnetic substance prevents the polishing process to suppress the effect of enhancing surface roughness. Other problems include scratching caused by fragments of removed material that become entrained in the magnetic fluid, and the fact that in the case of a magnetic workpiece, movement of the abrasive particles is constrained, making it impossible to achieve the required polishing effect. Such problems have limited the application of polishing using magnetic fluids.
Magnetically responsive (MR) fluids are fluids containing a dispersion of iron powder or other such ferromagnetic particles having micrometer-order sizes. When a magnetic field is applied to such a fluid that also contains abrasive particles, it sets up a strong interparticle attraction that causes the ferroparticles to rapidly aggregate into thick magnetic column formations that can be used to apply a powerful processing force to the workpiece surface, ensuring a high processing efficiency. However, because it has been considered difficult to control the shape, position and apparent viscosity of the formations, it is thought that there is a high risk that applying pressure to the formations will produce scratching of the workpiece surface. Thus, it has been considered that these MR fluids are usable for primary rough grinding but are not readily usable for fine and finish polishing. The large size of the iron particles generally used in the MR fluids has posed another obstacle to their use for fine and finish polishing.
FIG. 12 shows an example of a prior art polishing apparatus. This apparatus includes a rotary electrode 1 and a polishing pad 2 directly under the rotary electrode 1. The abrasive comprises abrasive particles dispersed in silicon oil or a lubricant having electrically insulating properties. Applying an alternating-current voltage between a conductive specimen 3 and the rotary electrode 1 causes the abrasive particles to be alternatively attracted to, and repelled by, the electrode. The electrode 1 is supported so that it can be rotated in the direction indicated by arrow A by a drive means (not shown).
FIG. 13 illustrates the effect of the prior art electrode. The application of an electrical field causes the abrasive particles 4 to cluster together into chain formations having a perpendicular alignment with respect to the conductive specimen 3. In this case, there is a mutual repulsion between adjacent clusters, and in order to maintain a certain spacing, the positioning of the abrasive particles can readily become uneven, giving rise to non-uniformity in the surface roughness of the polished surface.
With the polishing apparatus of the prior art thus configured, the movement of the abrasive particles from the rotary electrode 1 onto the conductive specimen 3, and the uneven positioning of the abrasive particles, makes it difficult to achieve a high-quality polished surface in the case of large products. Moreover, having to apply a voltage between the rotary electrode and a conductive workpiece makes it difficult to apply the apparatus to insulating materials. That is, the thickness of a workpiece having insulation properties has the same effect as an air-gap, so a high polishing effect cannot be obtained without using an electrical field strength that is high enough to control the position of the abrasive particles, which means the work is dangerous and there is a risk of the workpiece being damaged by an electrical discharge.
Also, centrifugal force generated by the rotation of the polishing part tends to cause the abrasive particles to accumulate around the periphery of the polishing area, reducing polishing efficiency. In response, prior art polishing apparatuses have sought to achieve a uniform distribution of abrasive particles with respect to the workpiece by again utilizing Coulomb force to return the abrasive particles to the polishing area, using the fact that the particles to which an electrical field has been applied have a high dielectric constant. However, the aforementioned spacing between adjacent clusters of abrasive particles has made it impossible to achieve a highly uniform arrangement of the particles.
An object of the present invention is therefore to provide a polishing apparatus that can polish insulative materials and can readily control the uniformity of the abrasive particle arrangement.